The cyanide ion can form a strong combination with the low heavy metal ion of the soft acid. Based on this, cyanide is widely used in the wet smelting of gold and silver. Cyanide has been used extensively for gold is mined, because elemental gold cyanide ion complexation due to reduced the oxidation potential and is oxidized by oxygen in the air to form soluble in alkaline conditions of gold salts and dissolve, thereby effectively separate the gold from the slag, and then active metal such as zinc block after the replacement reaction to gold from solution reduced to metal (see hydrometallurgy).
4Au+8NaCN+2H2O+O2=4Na[Au (CN) 2]+4NaOH
2Na[Au (CN) 2]+Zn=2Au+Na2[Zn (CN) 4]